This invention relates to an improved electrical system wherein compensation is provided for a sensor signal to which an electronic circuit responds so that variations in current within the system do not effect the response of the electronic circuit to the sensor signal. The invention is particularly suitable for use in conjunction with a motor vehicle electrical system.
It has become general practice in the design of motor vehicles to provide storage batteries as a source of DC electrical energy, starter motors supplied with energy from the DC source, and alternator systems for charging the DC source during normal operation of the engine. Typical reference potential or grounding practice provides a conductive strap that interconnects the negative terminal of the battery with the vehicle's engine block, thereby, providing a "common" or "ground" reference potential throughout the motor vehicle.
In recent times, the application of electronic devices to motor vehicles has increased. Electronic control units associated with engine or other vehicle systems are supplied with sensor signal voltages representing parameters of engine or vehicle operation. Some of these sensors provide very low voltage signals that are used by digital electronic circuits which achieve some desired control function.
One such sensor is an exhaust-gas-oxygen (EGO) sensor that is mounted in the exhaust conduit from the engine and that provides a signal in the millivoltage range representative of the partial pressure of oxygen in the exhaust stream emanating from the engine. When the mixture of air and fuel supplied to the engine is rich, the EGO sensor provides a voltage in the range from about 800 to 900 millivolts, and when the mixture supplied to the engine is lean, the sensor output voltage is in the zero to 100 millivolt range. A rapid transition between the two voltage levels occurs when the mixture supplied to the engine changes from rich to lean and vice versa.
The EGO sensor typically is of the zirconia type commercially available and is threadedly connected to the exhaust conduit from the engine, which exhaust conduit is made from a conductive material, and the sensor has a single lead wire from it connected to an electronic unit controlling the air-fuel ratio supplied to the engine. The reference potential for the sensor is a terminal attached electrically to the exhaust conduit at a location near the EGO sensor. This connection is coupled to the electronic circuit associated with the sensor.
Unfortunately, during cranking of the engine and during engine operation, the current flowing between the engine block and the negative or source reference terminal of the DC source of electrical energy, the battery, is substantial. For example, during engine cranking, this current may be 40 amperes or more and a small resistance between the engine block and the negative terminal of the battery of, for example, 0.20 ohms is sufficient to produce an 800 millivolt voltage drop across the ground strap or cable interconnecting the negative terminal of the battery and the engine block. Since the negative terminal of the battery typically is directly coupled to the electronic control unit associated with the EGO sensor and since there is a variable voltage drop in the ground strap interconnecting the engine block and the negative terminal of the battery, the sensor voltage signal may be completely lost, may be ignored by the electronic circuitry associated with the sensor, or may be of a level such that the electronic circuit cannot detect voltage levels or transitions between voltage levels.